Hewlett-Packard HP omnigo 100LX hard reset: Immediately after turning on the machine press and...

HP Omnigo 100 / 120     Press the RESET button on the bottom of the device.     Remove the main and backup battery for five minutes or more.

Soft Reset: SHIFT+ON+PgDn

System Reset: Immediately after turning on the machine press and hold the 'C' key for a second.
You'll be asked to initialize the RAM. Press 'Y' to initiate a Hard Reset, which erases all data in RAM.
If you press 'N' instead the machine will try to do a normal restart.